Hydraulic reciprocating machines are well known with most including a piston which moves sealingly within two or more hydraulic chambers. The piston typically has a stepped diameter which defines differential areas or lands on which the hydraulic fluid pressure can act. At least one chamber of the machine is supplied with liquid at supply fluid pressure. At least one of the other chambers is alternately supplied with supply pressure liquid or is isolated from the supply and is open to an exhaust path to a lower pressure or preferably to atmosphere, as the piston is reciprocated in the two chambers. The fluid access to the second chamber is controlled by inlet and exhaust valves. The differential area of the piston coupled with means for opening and closing the inlet and exhaust valves at appropriate times results in a cyclic reciprocating piston motion. Various valve arrangements and means for causing the valves to operate are known in the art. Hydraulic actuators based on the use of spool valves have been in wide use for decades. Actuators based on the use of poppet valves are, however, a more recent innovation.
In an early form of poppet operated actuator such as that described in U.S. Pat. No. 4,450,920 it was proposed that the drop in drive chamber pressure resulting from the closure of the inlet valve during a cycle be used to open a biased exhaust valve. Further, it was proposed that the flow of fluid into the drive chamber on the opening of the inlet valve be used to close the exhaust valve. This arrangement in practise is wasteful of high pressure supply fluid and frequently suffers from severe cavitation and erosion across the face of the exhaust valve.
An improved actuator is described in South African patent No. 84/9716 which uses the return motion of the piston, more particularly an interaction face on the piston, to open the inlet valve and in addition position part of the exhaust valve between the piston interaction face and the inlet valve in such a way as to ensure the closure of the exhaust valve prior to the opening of the inlet valve. In this manner the wasteful loss of supply fluid, characteristic of the above American patent is overcome. The exhaust valve of the actuator described in the specification is mechanically opened by using a "lost motion arrangement" which shortens the exhaust valve stroke to only marginally less than that of the piston. Problems have arisen in practise with the actuator described in the South African patent in that the exhaust valve stroke is substantially linked to the piston stroke and as a result the valve, on opening, travels at very high velocities and is frequently destroyed mechanically as a result. Additionally, the exhaust valve kinetic energy dissipation against the inlet valve causes undesirable bounce of the inlet valve from its seat, and consequently inefficiency.